What does the shift solenoid do in a 99 Honda Accord Ex?

I have a 99 Honda Accord that stops pulling sometimes. I have been to the honda dealerships and they say that my transmission is fine but I may need a shift solenoid. I have been looking at at shift solenoids and I see that they are a expensive but key part for the honda. Before I buy this part I want to know what exactly it does for the car. I have the Accord with a Vtec 3.0L 4cyl